When a cooling operation and a heating operation are switched in an air conditioner, a coolant flow is switched by using a four-way valve. In general, a switching method of the four-way valve is applied with respect to one side piston room and the other side piston room at both-end portions of the four-way valve, and a high-pressure coolant is supplied to the one side piston room via a control solenoid valve, and the coolant is aspirated from the other side piston room via the control solenoid valve so as to decrease a pressure of the coolant, and a valve unit is operated in accordance with a generated differential pressure (a detail explanation is described later).
A conventional vehicular air conditioner is disclosed in Patent Document 1. The conventional vehicular air conditioner is related to a switching method of a four-way valve, and when a switching command of a cooling operation and a heating operation is received while a compressor is stopped, the vehicular air conditioner is controlled in such a way that the four-way valve is switched after a thermostat is turned on, and an inverter frequency exceeds a predetermined value. Moreover, when the switching command of the cooling operation and the heating operation is received while the compressor is operated, the vehicular air conditioner is controlled in such a way that the thermostat is turned off after the four-way valve is switched. Thereby, it is eliminated that a differential pressure, which is required for a switching operation of the four-way valve, is not sufficiently generated.